Laminating process



Patented May 22, 1934 UNITED STATES PATENT OFFICE 1,960,177 I LAMINATING raocsss Josef Weber and Franz Hengstebeck, Essen, Germany, assignors, by mesne assignments, to Th. Goldsclimidt Corporation, New York, N. Y., a corporation of Delaware No Drawing. Original application June 2, 1931,

Serial No. 541,734.

Divided and this application March 17, 1932, Serial No. 599,586. In Germany February 2, 1929 1 Claim. (cam-s09) This invention relates to laminating processes; and it comprises a method of making improved laminated articles such as plywood, wherein a sheeted adhesive is assembled between layers of material, such as wood, heat and pressure are applied to the assemblage thereby softening and fusing said adhesive to form a mobile liquid and the said adhesive is then allowed to harden in situ to unite said layers with a hard insoluble bond; the said sheeted adhesive comprising an open textured reinforcing paper sheet impregnated with a dried, potentially reactive condensation product of an aldehyde with a phenol fusible at temperatures not substantially exceeding 130 C. to give a homogeneous, easy flowing liquid and said condensation product being formed in the presence of aqueous caustic alkali sufflcient to maintain the products in solution during and plywood, the amount of water'to be dissipated in the glue used may be as high as 30 or 40 per cent of the weight of the wood to be joined. In building up plywood assemblages, the glue is applied to inner layers and the water is taken up by proximate layers thereby necessarily swellingthese. At this stage in the operation the product includes a swollen inner wood layer connected to less moist outer layers or plies. In drying, the swollenirmer layers tend to shrink to their original proportions thereby setting up hgavy stresses and causing warping. Making plywood in this manner by the assemblage of successive laminae of wood with in: termediate drying is a tedious operation and the product is apt to be irregular. The whole assemblage must be finally dried and unless this final drying, as well as the intermediate dryings, be conducted with exceptional care and skill an unsatisfactory product results. It may be warped, mottled or exhibit fissures (incipient cleavage). In order to obviate these difiiculties it has been proposed to manufacture plywood and aflix veneers by employing between the layers of wood a cementing layer in the form of glue-sur faced sheets of paper, of fabric, or even of wood.

In making the cementing layer, suitable sheets are passed through a glue solution andafterwards dried so as to form adhesive surfaced sheets adapted to be interposed between the wood plies. 'Using sheets of this character the amount of water necessary can be limited to that required to make the surface of the sheets sticky; and this is an advantage. This method while better than the more common procedure of cementing by liquid glue'directly applied is nevertheless opeito,

certain objections. For one thing in the final product, there is an intermediate structural element between the wood plies. For another, plywood made in this manner when subjected to changes in temperature and moisture conditions frequently tends to split apart along the plane of the intermediate fabric layer; the layer carrying the glue on its surfaces. Unless the glue is waterproofed in some manner, the dried layer is not resistant to the action of moisture.

In aflixing veneers and in making plywood it is desirable to have the wood layers as close together,

face to face, as possible, restricting the amount of intermediate adhesive to that suflicient to even out surface irregularities. And it is desirable to have an adhesive'requiring in setting the dissipation of no great amount of water or of a volatile solvent; of any amount causing substantial swells ing in the wood. These results are accomplished in the present invention wherein is provided a thin substantially dry, substantially homogeneous lamina of adhesive usually containing a tenuous fiber reinforcement in negligible amount; the adhesive being solid at the ordinary temperature,

softening and fusing to a mobile liquid at higher temperatures and finally setting to a hard infusible, permanent form on exposure'to temperatures such'as are used in making plywood, say at For our purpose we use a particular condensation product of a phenol and formaldehyde in the presence of a rather large amount of alkali; this alkali serving both as a catalyst in inducing condensationand having certain other functions. As the phenol, ordinary phenol may be used or one of the cresols. In lieu of formaldehyde, acetaldehyde or other aldehydes maybe employed. However, ordinary phenol and ordinary formaldehyde, as the commercial 40 per cent solution, give good results. On dissolving equivalent amounts of phenol and formaldehyde (in solution) in 40 per cent caustic soda lye using the amount of lye necessary for solution, a satisfactory preparation can be made for the present purposes. With a batch of 94 kilograms of phenol and 100 kilograms of commercial 30 per cent formaldehyde solution about 17 kilograms of 4 0 per cent NaOH solution give a. good preparation.

This liquid is kept warm for a time say at 80 C.

for 1 hours, with constant stirring until an easy flowing oily dark liquid is obtained. The viscosity depends upon the time of heating but is rather low inany event. The exact viscosity desired can be obtained by interrupting the heating and then cooling. Because of the solvent influence of the excess of alkali and of the water present, no separation into two layers is produced. The oily water. terial may be made in thin sheets. These sheets liquid can be dried to a substantially dry material at temperatures below about 100 C. without going into the final insoluble resin. Ordinarily it is desired to dry the material to about 2 per cent By proper manipulation the dried maat the ordinary temperature are not tacky but are somewhat hygroscopic in the sense that they will take up more water from the air. Ordinarily the amount taken up will give a final product of about 5 to 8 per cent; this varying with the humidity. With this content of water the prodnot is flexible and not sticky or tacky and the sheets can be packaged, shipped and stored as rolls without danger of adhesion.

As so far described, the sheets aremade without ner; a web of paper passing first through the initial condensation product, past equalizing rolls and then-through drying apparatus. Dryingis usually so conducted as to give about 2 per cent water in the dry material and suificient exposure to air is then permitted to allow assumption of the full amount of water of condition; usually about 8 per cent. The proportion of fiber to adhesive varies but is usually in about the ratio of 1:2. With thin open textured paper in this proportion the final sheeted product has a substantially continuous body of adhesive. Inmaking such completely and uniformly impregnated sheets which are in fact continuous films of adhesive, the thickness and porosity or open texture of the reinforcing sheet are so correlated with the liquidity or viscosity of the oily liquid condensation product containing caustic soda as to insure a complete, continuous and uniform impregnation. When this relation is properly established the oily liquid penetrates all the pores of the reinforcing medium and it is only necessary to apply the proper amount of liquid to give the desired ratio of condensation product to the reinforcing material.

The presence of the caustic soda, (for which caustic potash may be substituted), in the composition in about the amount indicated gives a preparation which at the ordinary temperature is not tacky; isnot hygroscopic enough to become moist and sticky in ordinary air. It does however take up a little water of condition; enough to make it flexible and easily handled. The dry composition on warming to a few degrees above the ordinary temperature becomes plastic enough to facilitate easy sheeting and on recooling is permanent; it can be stored, packaged and handled without trouble. In heating to fusion in use, probably the caustic soda and the small amount of water of condition present exercise a solvent action or contribute -to fusion. The

caustic soda solution produces condemation in making the initial oily liquid but it does not cause condensation much beyond this point with any reasonable degree of temperature until the heat reaches a point somewhere around 130 0. when the catalytic action becomes extremely rapid. In

the final product the caustic soda does not exist in any form' making the article hygroscopic; nor is it necessary in making plywood to provide for the dissipation of the water present in the mixture. It is, in any event, small in amount being only about 5 to 8 per cent of the adhesive.

When the flexible sheeted adhesive (with or without reinforcement) is placed between two layers of wood in the usual way of aflixing veneers or making plywood and heat and pressure are applied, the adhesive at first yields and softens, then fuses and is then taken up for the most part by the pores of the wood; finally solidifying in place. With about 8 per cent water in the material on fusing it forms a fairly mobile liquid readily taken up by pores as a whole; there being no segregation of solvent by the pores of the wood as there usually is with moist glue made plastic by heat. The alkali and water have a fluxing action but the melt is homogeneous. In the presence of the alkali and water, there is some re--.

action on the wood fiber contributing to the extremely perfect cohesive union ultimately secured.

0n heating the assemblage of plies and adhesives up to, say, 130 C. the condensation products are converted into the final insoluble resin without much changein volume and with very little extrication of water; not enough to cause swelling or require any special care in subsequent dryi g Because of the properties of the presentadhesive sheet it is possible to join a number of layers of plywood together simultaneously instead of successively as is the usual practice; thereby economizing time and labor.

While the invention has been particularly described in its usein cementing veneers and making plywood, the adhesive sheets of the present invention are advantageous in many other relations and may be used for general cementing purpose. Their use in forming a facing layer on various articles is advantageous This application is a continuation-in-part of our application,-Serial No. 424,715, filed Jan. 30, 1930 and a division of our application; Serial No. 541,734, filed June 2, 1931.

What we claim is:

In the manufacture of plywood articles having at least two layers of wood united by an interposed hardened insoluble adhesive in interlocking engagement therewith, the process which comprises warming a phenol and formaldehyde together in the presence of an aqueous caustic soda solution under conditions suitable for producing a'homcgeneous, oily, liquid condensation product without the separation of an aqueous layer, impregnating a porous, open textured, thin tissue paper sheet with said product and drying the impregnated paper; the amount of alkali present not being sufficient to give the dried sheet a tacky surface but being sufiicient to cause absorption of from about 5 to 8 per cent of water of condition on exposure. to air; assembling the rereinforced sheet of adhesive between layers of wood, applying pressure to said assemblage while heating to a temperature not substantially exceeding 130 C. thereby softening and fusing said adhesive to form a mobile liquid and then hardening said adhesive in 'situ to unite said layers with a hard insoluble bond.

J OSEF WEBER. FRAIQZ HENGSTEBECK. 

